An atom is the smallest particle of an element that can exist. Atoms combine to form molecules, which are the basic units of compounds and some elements. Molecules can be represented by chemical formulas that show the number and type of atoms in the molecule, such as H2O for a water molecule containing two hydrogen atoms and one oxygen atom. Circles and spheres are also used to model atoms and molecules to aid in the study of their structures.

1. ATOMS AND
MOLECULES
CHEMISTRY
MADE BY SALAR ALI

2. What is an atom?
• An atom is defined as the smallest particle of an element that can exist.
• Atoms are the building blocks of all matter.
• All matter, living or non-living, is made up of atoms.
• Atoms are so small that we cannot see them without the help of electron
microscopes.
• Each element is made up of its own type of atom.
• The atoms of different elements are different in sizes.

3. Representing atoms
• Circles are commonly used to represent
atoms.
• Another way of representing different kinds
of atoms is by using spheres of various sizes
and colours.
• Representation of
atoms using models
and diagrams makes the study and
understanding of atoms and molecules.
Na

4. What is a molecule?
• A molecule is made up of two or more atoms of the same kind or of
different kinds chemically combined together.
• Molecules exist in both elements and compounds.
• Molecules in elements consist of a fixed number of one kind of atom only.
• In some cases, three identical atoms may combine to form a molecule, as
in the ozone molecule which is made up of three oxygen atoms.
• Molecules in compounds consist of a fixed number of two or more
different kinds of atoms chemically combined together.

5. Chemical Formula
The atomic composition of a water
molecule
can be described either
• In words, i.e.
‘Each molecule of water contains two
atoms of hydrogen and
one atom of oxygen chemically
combined together.’
or
• by its chemical formula, i.e.
H₂O
• The chemical formula of a molecule
shows the number and kinds of atoms
contained in it.
• The chemical formula of a molecule of
an element is denoted by the chemical
symbol of the element and a subscript
showing the number of atoms in the
molecule.
• For example, H₂ denotes a molecule of
hydrogen which consists of two
hydrogen atoms.
• Let us take a look at some examples on
the next slide.

6. EXAMPLES
CHEMICAL FORMULA MEANING
CO
Every carbon monoxide molecule contains
one carbon atom and one oxygen atom
chemically combined together.
H₂O
Every water molecule contains two
hydrogen atoms and one oxygen atom
chemically combined together,
C₁₂H₂₂O₁₁
Every sucrose molecule contains 12 carbon
atoms, 22 hydrogen atoms and 11 oxygen
atoms chemically combined together.